This invention relates to testing adhesion of a thin film on a substrate.
Thin surface coatings are deposited in various ways and serve diverse purposes such as providing resistance to wear and corrosion, reducing friction, providing specialized optical properties, or to obtain desirable electrical or magnetic attributes. It is desired to adequately bond the thin film to its substrate in order to accomplish their design intent. Adhesion between the film and its base material is the limiting factor to operational usefulness. For example, diamond thin films are very hard but its adhesion to steel is limited due to the interfacial adhesion. Similarly, cubic-BN is nearly as hard as diamond but the hexagonal-BN phase that forms as the initial substrate-film interface has very poor adhesion.
Adhesion is a function of interfacial bond strength and a host of other factors. The force or the work required to detach in situ a given thin film coating from its substrate indicates adhesion strength. In The Materials Science of Thin Films, Milton Ohring (1992), “ASTM defines adhesion as the condition in which two surfaces are held together by valance forces or by mechanical anchoring or by both together.” Adhesion to a substrate is the first attribute a film must possess before any of its other properties can be successfully exploited. The lack of a broadly applicable method for quantitatively measuring adhesion makes it virtually impossible to test the efficacy of prior techniques. The ACIM method of assessing thin film integrity is expected to fulfill this requirement.
Several methods are currently used to indirectly measure thin film adhesion. These include the scratch test, peel test, and indentation test. In the scratch test, a smooth but finely pointed stylus is drawn across the film under increasing vertical loads. The critical load required to strip the film from its substrate and leave a clear channel indicates adhesion strength. In the peel test, thin film adhesion is measured by the tensile force required to pull the film directly from its substrate. In the peel test or the tape test, adhesive tape is pressed onto the thin film and then torn off. The quality of adhesion is judged by whether the film is pulled off with the tape. In the indentation test, which can be called an impression test, forces imposed through an indentation to introduce a residual stress field that causes the interface to delaminate. Based on the measurements of the force and the size of the induced delamination one can estimate the interface toughness. Impression tests are typically used on ductile substrates having well established stress/strain characteristics. Other specialized tests include the bulge test and the blister test, which require very special sample preparations. Nano indenting is a new method for characterizing mechanical properties on micro-scale-features less than 100 nm across and thin films less than five nm thick.